I'm looking for an alternative under Ubuntu for a software that I used to have long time ago under Windows. Its called TextAluod. It has natural voices that you can use. So far, I tried ESPEAK through terminal, but is too robotic, so I would like to know which alternatives are available for our dear Ubuntu and how natural the voices are?

Cepstral has some really nice voices, but they're $30 each. They're designed to be used with IVRs, but they can behave like espeak by typing padsp swift "Some text" from the command line. There is a demo of the voices on the website.

They do have some for 'personal use', but they seem to be a full version behind the ones for professional use. Not sure if they require payment as well, but it's worth checking out.

I would check out http://www.spokentext.net/. I found it very good. I have a lot of old writings that I wanted to make into Audio book and this site lets me do this. You have your choice of many different voices (Male and Female). Worth taking a look at.